The present patent application claims the right of priority under 35 U.S.C. xc2xa7119 (a)-(d) of German Patent Application No. 100 63 296.3, filed Dec. 19, 2000.
The present invention relates to a process for producing carbonic acid diaryl esters by reacting monophenols and phosgene in an inert solvent in the presence of alkali and a nitrogen base in an interfacial process at low reaction temperatures.
An improvement to the interfacial boundary process for producing carbonic acid diaryl esters is disclosed. In the two-stage process that entails phosgene and monophenols reaction in an inert solvent, in the presence of alkali and in the presence of a nitrogen base catalyst, the improvement comprising maintaining a temperature below 50xc2x0 C. in the first and second stages.
The production of carbonic acid diaryl esters by the phase interface process is basically known from the literature, cf. Chemistry and Physics of Polycarbonates, Polymer Reviews, H. Schnell, Vol. 9, John Wiley and Sons, Inc. (1964), p. 50/51). U.S. Pat. No. 4,016,190 describes a process for the production of carbonic acid diaryl esters which is carried out at temperatures  greater than 65xc2x0 C. and in which nitrogen bases are used as catalysts.
Aryldialkyl urethanes are typically formed as secondary products when carrying out these processes, either due to the reaction of phosgene with the tertiary amines used as catalysts to form carbamic acid chloride and subsequent reaction with the phenol present to form the urethane (1) or due to the reaction of aryl chlorocarbonic acid ester from phenol and phosgene with the tertiary amines (2) in accordance with the following general reaction scheme: 
wherein R represents the radicals according to formula (I) (see below) and Rxe2x80x2, Rxe2x80x3 and Rxe2x80x2xe2x80x3 generally represent organic radicals.
These compounds interfere with the melt transesterification process to the polycarbonate and are concentrated, in particular, in a carbonic acid diphenyl ester cycle. It is therefore particularly important to suppress the formation of these Aryldialkyl urethanes and to remove them from the carbonic acid diaryl ester.
However, the processes known from the state of the art, based on monophenols and phosgene in a solvent and using a nitrogen base as catalyst are not satisfactory with respect to the purity of the products obtained, in particular with respect to the chlorine and urethane content thereof. For this reason, the products obtainable by these processes additionally have to be subjected to further purification processes.
Starting from the state of the art, the object was accordingly to provide a process for producing carbonic acid diaryl esters which are depleted in chlorine and, in particular, in secondary products, especially in urethane.
It has now surprisingly been found that, when producing carbonic acid diaryl esters by reaction of monophenols and phosgene in an inert solvent by the phase interface process in the presence of alkali while using nitrogen based catalysis, low reaction temperatures are required for obtaining high product purity in the carbonic acid diaryl ester and for avoiding the aforementioned secondary reactions.
The catalysts to be used in the process according to the invention are tertiary amines, N-alkyl piperidines or onium salts. Tributyl amine, triethyl amine and N-ethylpiperidine are preferably used. N-ethylpiperidine is quite particularly preferred. The concentrations of the catalysts are 0.0001 mol % to 0.1 mol %, preferably 0.01 mol % to 0.075 mol %, based on the phenol used.
The suitable alkali include lye (Na, K, Li, Ca-hydroxide), preferably sodium hydroxide solution, and is preferably used as a 20 to 55 wt. %, particularly preferably 30 to 50 wt. % solution in the process according to the invention.
Phosgene may be used in liquid or gaseous form or dissolved in the inert solvent.
Suitable monophenols for use in the process according to the invention include phenols of formula (I) 
wherein
R is hydrogen, tert.-butyl, halogens or a branched or unbranched C8 and/or C9 alkyl radical,
therefore phenol itself, alkylphenols such as cresols, p-tert.-butylphenol, p-cumylphenol, p-n-octylphenol, p-iso-octylphenol, p-n-nonylphenol and p-iso-nonylphenol, halogen phenols such as p-chlorophenol, 2,4-dichlorophenol, p-bromophenol and 2,4,6-tribromophenol may be used. Phenol is preferred.
Inert organic solvents, for example dichloromethane, toluene, the various dichloroethanes and chloropropane compounds, chlorobenzene and chlorotoluene may be used in the process according to the invention, dichloromethane preferably being used.
The reaction may be conducted both discontinuously (batchwise) and continuously, preferably continuously, and is preferably carried out in a plug flow without significant back-mixing. This may be effected, for example, in tubular reactors. The two phases (aqueous and organic phase) may be thoroughly mixed by diaphragm fittings, static mixers and/or pumps.
The reaction initiated by combining the phosgene, the inert solvent, which preferably serves only as solvent for the phosgene, and the phenol, which has preferably already been dissolved in the lye beforehand. Generally, the pH of the first stage in the process according to the invention is preferably adjusted by the ratio of lye/phenol/phosgene in such a way that the pH lies in the range of 11.0 to 12.0, preferably 11.2 to 11.8. The reaction temperature is kept at  less than 50xc2x0 C., preferably  less than 40xc2x0 C., quite particularly preferably  less than 35xc2x0 C. by cooling. The residence time lies in the range of 2 seconds to 300 seconds, preferably in the range of 4 seconds to 200 seconds in the first stage in the case of the continuously conducted process according to the invention.
The reaction is completed to form the carbonic acid diaryl ester in the second stage of the process according to the invention. The catalyst is preferably provided at this stage. The reaction mixture is cooled in the process according to the invention directly after or during addition of the catalyst. The reaction temperature is kept at  less than 50xc2x0, preferably  less than 40xc2x0 C., quite particularly preferably  less than 35xc2x0 C. by cooling. It may be advantageous to add the catalyst at a plurality of, preferably two, points of the second stage of the process.
The chloroformic acid aryl ester content in the resultant carbonic acid diaryl ester is  less than 2 ppm, preferably  less than 0.5 ppm in the process according to the invention.
An arylpiperidyl urethane content in the resultant carbonic acid diaryl ester of  less than 100 ppm, preferably  less than 75 ppm, quite particularly preferably  less than 50 ppm is obtained by the process according to the invention.
The pH is preferably regulated in the second stage of the process according to the invention by measuring the pH (which is preferably measured online in the continuous process) and corresponding adjustment of the pH by addition of the lye. The quantity of lye supplied is adjusted in such a way that the pH in the second stage of the process lies in the range of 7.5 to 10.5, preferably 8 to 9.
In the process according to the invention, phosgene is supplied in a ratio of 1.01 to 1.15 mol %, preferably 1.05 to 1.12 mol %, based on the phenol. The solvent is added in such a way that the carbonic acid diphenyl ester exists in a 5 to 60% solution, preferably 20 to 45% solution after the reaction.
After the reaction, the organic phase containing the carbonic acid diaryl ester is usually washed with an aqueous liquid and separated as far as possible from the aqueous phase after each washing procedure. Washing is preferably carried out with de-ionized water. The carbonic acid diaryl ester solution is usually cloudy after washing and separation of the washing liquid. Aqueous liquids, for example dilute mineral acids such as HCl or H3PO4 are used to separate the catalyst and water which is completely de-ionized is used for further purification. The concentration of HCl or H3PO4 in the washing liquid may be, for example, 0.5 to 1.0 wt. %. The organic phase is preferably washed twice by way of example.
Basically known separating vessels, phase separators, centrifuges or coalescers or also combinations of these devices may be used as phase-separating devices for separating the washing liquid from the organic phase.
The solvent is evaporated to obtain the high-purity carbonic acid diaryl ester. Evaporation may be carried out in a plurality of evaporator stages. For example, it is carried out through one or more successive distillation columns in which the solvent is separated from the carbonic acid diaryl ester.
This purification stage or stages may be conducted, for example, continuously in such a way that the sump temperature during distillation is  greater than 150xc2x0 C. to 310xc2x0 C., preferably  greater than 160 to 230xc2x0 C. The pressures required to carry out these distillation processes are between 1 and 1000 mbar, preferably between 5 and 100 mbar.
The resultant carbonic acid diesters are distinguished by very high purity with GC purities (cool on column method)  greater than 99.99%, preferably 99.9925%, quite particularly preferably  greater than 99.995% and extremely good transesterification behaviour, so a polycarbonate of excellent quality may be produced therefrom.
The production of aromatic oligo/polycarbonates by the melt transesterification process is known from the literature and has already been described, for example, in the Encyclopedia of Polymer Science, Vol. 10 (1969), Chemistry and Physics of Polycarbonates, Polymer Reviews, H. Schnell, Vol 9, John Wiley and Sons, Inc. (1964) or U.S. Pat. No. 5,340,905.